1. Field of the Invention
The present invention is directed to the manufacture of integrated circuits and, in particular, to a method and system for restoring transverse magnetic wave contrast in features made by a lithographic process.
2. Description of Related Art
Manufacturers of microelectronic circuits are continually seeking to produce features having smaller dimensions. The lithographic production of such features typically uses a step-and-scan imaging tool 20, as shown in FIG. 1, to project a pattern onto a photosensitive resist layer on a substrate or wafer. The projection optical system of the imaging tool includes a lamp, laser, or other optical source 22 that projects radiation 24 used to illuminate a photomask or reticle 28 through a condenser lens system 26. The photomask or reticle 28 contains the pattern to be projected and reproduced on the wafer substrate, and is generally oriented substantially perpendicular to an optical axis 24 of the projection optical system. Some of the light radiation 46 that passes through the photomask 28 is collected by the projection optics 34 and the aerial image 36 of the pattern produced by passage of radiation 46 through the mask is directed onto the wafer 42, so as to create the pattern or image 40 on the wafer.
In a step-and-scan system, the photomask 28 and the wafer 42 are mounted on mask stage 33 and substrate stage 38, respectively, that move relative to the fixed optical system. The optical system contains an aperture or slit 32 through which light is allowed to pass to the reticle. The entire mask pattern within the desired transfer region of a reticle is completely exposed by scanning along the one-dimensional scan direction 30 and across the complete one-dimensional width of the transfer region to produce a complete pattern 40 on the wafer resist, for example a complete chip pattern. The scanning process is subsequently repeated to produce the desired number of patterns on the wafer 42.
In order to produce features having smaller dimensions in the manufacture of microelectronic circuits, three factors, a phenomenological process resolution factor (k1), the light wavelength (λ) and the numerical aperture value (NA) are involved in the lithographic processing that may be used to create the minimum line width (Wmin) according to a standard generalization of Rayleigh's equation:Wmin=k1λ/NA 
Sometimes a slightly different value of k1 is used that relates λ and NA to the half-pitch of a periodic system of lines and spaces.
To enable use of finer features in integrated circuits many advances of been made in lithographic technology that allow smaller values of k1. In the early days of integrated circuit manufacture only k1 values above 1 were practical, but now k1 values of 0.4 are being employed, and further reductions are sought. A difficulty here is that image contrast is degraded at such low k1 values, making it difficult to achieve size uniformity in the printed circuit features as distributed over the chip, such size uniformity usually being required for acceptable circuit performance.
Looking at the NA (numerical aperture) factor, recent advances have enabled exposure tool manufacturers to ship tools with NA values in excess of 0.70, 0.80 and higher, and tools with NA values of 0.93 are now available. NA values higher than 1.0 are expected in the future, based on immersion imaging. Because modern exposure tools have such high NA values, images must be formed using waves with high angles of propagation within the resist, i.e., large propagation angles with respect to a direction normal to the surface of the resist layer. Such high angles of propagation may be considered to be those in excess of about 30°, since the orientations of the associated electric field vectors will then show significant variation, in that, for example, the electric field of a ray that has propagated from one side of the lens aperture into the resist can differ in its orientation by as much as 60° from the electric field of a ray that has entered the resist after exiting the opposite side of the lens aperture. It should be noted that resist refractive indices are usually greater than about 1.6, so that a propagation angle of 30° within a resist layer corresponds to an incidence angle for the incoming wave above the resist that is greater than 50° (NA=0.8) if the incident medium is air. Thus, incidence angles greater than about 50° may be considered to be high incidence angles when the incident medium is air, depending on the resist index. However, in an immersion system, where the incident medium might have a refractive index of 1.4, incidence angles greater than about 35° could be considered to be high incidence angles, since they too would produce a propagation angle of 30° or more within a resist layer, depending on the resist index.
At the high numerical apertures that produce such incidence angles, it has been observed that there is a fundamental loss of image contrast for the transverse magnetic (TM) polarization of the light waves. Even if the source radiation can be transverse electric (TE) polarized for the dominant interfering orders, i.e., from a tangentially polarized source, other orders will generally be present that interfere with fields that are partly TM polarized.
In FIG. 2, there is shown the passage of high incident angle light waves through resist layer 50 on either side of a line 47 normal to the surface of the resist layer. As used herein, the term “light” refers to the radiation used in the lithographic imaging system, regardless of wavelength. Incident waves 46a and 46′a enter the resist layer at opposite angles θ, and are refracted as waves 46c, 46′c, respectively. Incident light rays 46a, 46′a carry a high resolution image, and so have a high incidence angle θ with respect to the direction normal to the surface of resist layer 50 disposed on the wafer. The direction normal to the substrate will be referred to as the z direction. The refractive index of a typical resist layer is generally greater than about 1.6 and often greater than about 1.7, in the range of about 1.6 to 1.8, while the incident medium has a refractive index that is usually 1.0 and in general less than the resist index, so that the angle of the refracted light rays within the resist is generally reduced, i.e., becomes more vertically oriented. The TM polarized electric field vectors from these incident light rays are shown by arrows 46b and 46′b. These electric field vectors are perpendicular to the propagation directions of their respective waves, as required by Maxwell's equations. These interfering electric field vectors are roughly anti-parallel, corresponding to a dark portion of the image.
During half the optical cycle, electric fields 46d and 46′d on the refracted waves are oriented at an angle with a partially downward direction within the resist layer. Because the vectors share a common z component (downward during the part of the optical cycle shown, upward during the other half of the optical cycle, and in general with the same sign), the two vector fields do not completely cancel when superposed, the z components being parallel rather than anti-parallel, and thus the dark portion of the image is not fully dark, and so contrast is reduced, which in turn reduces the controllability of the imaging process, and thus increases the minimum practical k1. The essential problem here is that the lack of complete destructive interference at high NA values results in reduced contrast from the TM polarization component. This is conventionally regarded as a problem that is inherent to high NA imaging, due to the geometry of the propagation angles involved. The associated contrast loss adds to the already significant contrast loss that arises even when the NA value is not large if one forms images at low k1, as is desirable to maximize resolution.
In a similar way, full constructive interference cannot take place in bright regions of a high NA image having nonzero TM component, because the TM polarized electric fields are not fully parallel when propagation angles are steep. This reduces image brightness in TM polarization.
Neither contrast nor brightness is degraded in TE polarization (not shown), in which the electric field is polarized perpendicular to the plane of incidence onto the wafer. However, the printing of circuit patterns generally causes the waves that are incident on almost every small region of the wafer to have planes of incidence that are oriented in many possible azimuths. This is because most circuit patterns are populated fairly densely with fine circuit features in more than one orientation. This makes it impossible to achieve pure TE polarization for all features using a single exposure, and multiple exposures significantly increase cost. Moreover, even if one could theoretically employ waves that contained only a single polarization component, such as TE, the polarization can be distorted in a number of ways, for example by residual birefringence in the mask substrate and lens, diffractive effects in the mask, and thin-film effects in the lens and wafer process films. These polarization distortions can in turn cause variations in the dose that is delivered to different features in the image, since TE and TM peak brightnesses differ. In general, when diverse fine features are present with different orientations, the proportion of TM polarization to TE polarization will vary, and since TE polarization exhibits complete constructive interference, the total delivered dose will vary as the proportion of TM polarization to TE polarization varies. Low k1 imaging is particularly sensitive to dose variations. The sensitivity to polarization distortion is reduced if unpolarized light can be employed, even though unpolarized light can be understood as an incoherent superposition of TM and TE polarized light. However, for successful printing using unpolarized light, it is desirable that both polarizations provide high contrast, and that the resist system not exhibit differential polarization sensitivity that would in effect remove the unpolarized condition.